Is !list.isEmpty() and list.size()>0 equal?

Question

I've seen code as below:

     if (!substanceList.isEmpty() && (substanceList.size() > 0))
    {
      substanceText = createAmountText(substanceList);
    }

Would the following be a valid refactor?

     if (!substanceList.isEmpty())
    {
      substanceText = createAmountText(substanceList);
    }

I would be grateful for an explanation of the above code and whether the second version may cause errors?

Answer 1

If in doubt, read the Javadoc:

Collection.isEmpty():

Returns true if this collection contains no elements.

Collection.size():

Returns the number of elements in this collection

So, assuming the collection is implemented correctly:

collection.isEmpty() <=> collection.size() == 0

Or, conversely:

!collection.isEmpty() <=> collection.size() != 0

Since the number of elements should only be positive, this means that:

!collection.isEmpty() <=> collection.size() > 0

So yes, the two forms are equivalent.

Caveat: actually, they're only equivalent if your collection isn't being modified from another thread at the same time.

This:

!substanceList.isEmpty() && (substanceList.size() > 0)

is equivalent to, by the logic I present above:

!substanceList.isEmpty() && !substanceList.isEmpty()

You can only simplify this to

!substanceList.isEmpty()

if you can guarantee that its value doesn't change in between evaluations of substanceList.isEmpty().

Practically, it is unlikely that you need to care about the difference between these cases, at least at this point in the code. You might need to care about the list being changed in another thread, however, if it can become empty before (or while) executing createAmountText. But that's not something that was introduced by this refactoring.

TL;DR: using if (!substanceList.isEmpty()) { does practically the same thing, and is clearer to read.

Answer 2

Actually, you can read the source code downloaded in the JDK:

/**
 * Returns <tt>true</tt> if this list contains no elements.
 *
 * @return <tt>true</tt> if this list contains no elements
 */
public boolean isEmpty() {
    return size == 0;
}

I think that this settles all the queries.